The present invention is generally related to a lighting device, a backlighting device, and a display device with improved efficiency. The invention can be applied to light emitting devices such as general lighting of phosphor lamp, lighting for signals or automobiles, backlight for LCDs, and displays.
Self-absorption in a lighting device generally occurs when light emissions stay within the device to excite other phosphors, and be absorbed or otherwise prevented from exiting the device, thus reducing performance (intensity) and efficiency. For example, white light is produced for the different applications using down converting phosphors together with suitable light engines. Typical spectral range for light engines now is about 390-460 nm, a near UV-blue spectral region. Two ways are known to produce white light using phosphor down converting process. One way is that a LED blue light engine interacts with so-called “yellow” type phosphor and superposition of the blue pumping light with the broad band fluorescence light emitted by the phosphor. To improve CRI or have warm white light (CCT in the range 2500-3500 K), a red phosphor may be additionally used. However, overlapping of phosphor excitation and emitting spectra, which is the main condition for the self-absorption, reduces the total luminance output. Another way uses RGB phosphor compositions excited by near UV spectra region 390-405 nm. It also has the similar self-absorption problem. As such, suppression of self-absorption remains a need in this field of technology.
Japan patent application 2004088604 discloses a vertical geometry of phosphor layers where phosphor emitting a light of the wavelength near to red color is arranged nearer to the exiting element, and blue phosphor is arranged farther away from the exiting element.
Similar approach is also described for two-component phosphor system in Japan patent application 2003380788. However, the approach cannot suppress self-absorption significantly because, for example, 50% of the emitted green light will be directed to a red phosphor layer, despite that the red phosphor layer is located closer to LED light engine.
Advantageously, the present invention provides a lighting device with increased total output luminance, which employs a novel geometry of wavelength converting members that prevents self-absorption of fluorescence emitted by a certain type of phosphor by other type of phosphor.